Abstract

The solution phase behavior of short, strictly alternating multiblock copolymers of type was studied using lattice Monte Carlo simulations. The polymer molecules were modeled as flexible chains in a monomeric solvent selective for block type . The degree of block polymerization and the number of diblock units per chain were treated as variables. We show that within the regime of parameters accessible to our study, the thermodynamic phase transition type is dependent on the ratio of . The simulations show microscopic phase separation into roughly spherical aggregates for ratios less than a critical value and first-order macroscopic precipitation otherwise. In general, increasing at fixed , or at fixed , promotes the tendency toward macroscopic phase precipitation. The enthalpic driving force of phase change is found to universally scale with chain length for all multiblock systems considered and is independent of the existence of a true phase transition. For aggregate forming systems at low amphiphile concentrations, multiblock chains are shown to self-assemble into intramolecular, multichain clusters. Predictions for microstructural dimensions, including critical micelle concentration, equilibrium size, shape, aggregation parameters, and density distributions, are provided. At increasing amphiphile density, interaggregate bridging is shown to result in the formation of networked structures, leading to an eventual solution-gel transition. The gel is swollen and consists of highly interconnected aggregates of approximately spherical morphology. Qualitative agreement is found between experimentally observed physical property changes and phase transitions predicted by simulations. Thus, a potential application of the simulations is the design of multiblock copolymersystems which can be optimized with regard to solution phase behavior and ultimately physical and mechanical properties.

Received 18 December 2007Accepted 12 March 2008Published online 24 April 2008

Acknowledgments:

Financial support of this work was provided by the National Science Foundation through a Graduate Research Fellowship to M.G. and by the NSF NIRT Center on Nanoparticle Formation. Additional support was provided by the Department of Energy, Office of Basic Energy Sciences (Grant No. DE-FG02-01ER15121). M.G. also gratefully acknowledges financial support from Merck and Company through a Doctoral Research Fellowship.